2. Global Carbon Cycle Overview

5. The dynamics of terrestrial ecosystems depend on interactions between a
number of biogeochemical cycles, particularly the carbon cycle, nutrient cycles,
and the hydrological cycle, all of which may be modified by human actions.
Terrestrial ecological systems, in which carbon is retained in live biomass,
decomposing organic matter, and soil, play an important role in the global
carbon cycle. Carbon is exchanged naturally between these systems and the
atmosphere through photosynthesis, respiration, decomposition, and combustion.
Human activities change carbon stocks in these pools and exchanges between
them and the atmosphere through land use, land-use change, and forestry, among
other activities. Substantial amounts of carbon have been released from forest
clearing at high and middle latitudes over the last several centuries, and
in the tropics during the latter part of the 20th century. [1.1.1.2]
1

Table 1: Global carbon stocks in vegetation and soil carbon
pools down to a depth of 1 m.

Biome

Area
(109 ha)

Global Carbon Stocks (Gt C)

Vegetation

Soil

Total

Tropical forests

1.76

212

216

428

Temperate forests

1.04

59

100

159

Boreal forests

1.37

88

471

559

Tropical savannas

2.25

66

264

330

Temperate grasslands

1.25

9

295

304

Deserts and semideserts

4.55

8

191

199

Tundra

0.95

6

121

127

Wetlands

0.35

15

225

240

Croplands

1.60

3

128

131

Total

15.12

466

2011

2477

Note: There is considerable uncertainty in the numbers given, because of ambiguity
of definitions of biomes, but the table still provides an overview of the magnitude
of carbon stocks in terrestrial systems.

6. There is carbon uptake into both vegetation and soils in terrestrial ecosystems.
Current carbon stocks are much larger in soils than in vegetation, particularly
in non-forested ecosystems in middle and high latitudes (see Table
1). [1.3.1]

7. From 1850 to 1998, approximately 270 (± 30) Gt C has been emitted as carbon
dioxide (CO2) into the atmosphere from fossil fuel burning and cement production.
About 136 (+ 55) Gt C has been emitted as a result of land-use change, predominantly
from forest ecosystems. This has led to an increase in the atmospheric content
of carbon dioxide of 176 (± 10) Gt C. Atmospheric concentrations increased
from about 285 to 366 ppm (i.e., by ~28%), and about 43% of the total emissions
over this time have been retained in the atmosphere. The remainder, about
230 (± 60) Gt C, is estimated to have been taken up in approximately equal
amounts in the oceans and the terrestrial ecosystems. Thus, on balance, the
terrestrial ecosystems appear to have been a comparatively small net source
of carbon dioxide during this period. [1.2.1]

8. The average annual global carbon budgets for 1980-1989 and 1989-1998 are
shown in Table 2. This table shows that the rates and
trends of carbon uptake in terrestrial ecosystems are quite uncertain. However,
during these two decades, terrestrial ecosystems may have served as a small
net sink for carbon dioxide. This terrestrial sink seems to have occurred
in spite of net emissions into the atmosphere from land-use change, primarily
in the tropics, having been 1.7 ± 0.8 Gt C yr-1 and 1.6 ± 0.8 Gt
C yr-1 during these two decades, respectively. The net terrestrial
carbon uptake, that approximately balances the emissions from land-use change
in the tropics, results from land-use practices and natural regrowth in middle
and high latitudes, the indirect effects of human activities (e.g., atmospheric
CO2 fertilization and nutrient deposition),
and changing climate (both natural and anthropogenic). It is presently not
possible to determine the relative importance of these different processes,
which also vary from region to region. [1.2.1
and Figure 1-1]

9. Ecosystem models indicate that the additional terrestrial uptake of atmospheric
carbon dioxide arising from the indirect effects of human activities (e.g.,
CO2 fertilization and nutrient deposition) on a global scale is likely to
be maintained for a number of decades in forest ecosystems, but may gradually
diminish and forest ecosystems could even become a source. One reason for
this is that the capacity of ecosystems for additional carbon uptake may be
limited by nutrients and other biophysical factors. A second reason is that
the rate of photosynthesis in some types of plants may no longer increase
as carbon dioxide concentration continues to rise, whereas heterotrophic respiration
is expected to rise with increasing temperatures. A third reason is that ecosystem
degradation may result from climate change. These conclusions consider the
effect of future CO2 and climate change on the present sink only and do not
take into account future deforestation or actions to enhance the terrestrial
sinks for which no comparable analyses have been made. Because of current
uncertainties in our understanding with respect to acclimation of the physiological
processes and climatic constraints and feedbacks amongst the processes, projections
beyond a few decades are highly uncertain. [1.3.3]

Table 2: Average annual budget of CO2 for
1980 to 1989 and for 1989 to 1998, expressed in Gt C yr-1 (error limits correspond
to an estimated 90% confidence interval).

1980 to 1989

1989 to 1998

1) Emissions from fossil fuel combustion and cement production

5.5 ± 0.5

6.3 ± 0.6a

2) Storage in the atmosphere

3.3 ± 0.2

3.3 ± 0.2

3) Ocean uptake

2.0 ± 0.8

2.3 ± 0.8

4) Net terrestrial uptake = (1) - [(2)+(3)]

0.2 ± 1.0

0.7 ± 1.0

5) Emissions from land-use change

1.7 ± 0.8

1.6 ± 0.8b

6) Residual terrestrial uptake = (4)+(5)

1.9 ± 1.3

2.3 ± 1.3

aNote that there is a 1-year overlap (1989) between the two decadal time
periods.bThis number is the average annual emissions for 1989-1995, for which data
are available.

10. Newly planted or regenerating forests, in the absence of major disturbances,
will continue to uptake carbon for 20 to 50 years or more after establishment,
depending on species and site conditions, though quantitative projections
beyond a few decades are uncertain. [1.3.2.2]

11. Emissions of methane (CH4) and nitrous oxide
(N2O) are influenced by land use, land-use change, and forestry activities
(e.g., restoration of wetlands, biomass burning, and fertilization of forests).
Hence, to assess the greenhouse gas implications of LULUCF activities, changes
in CH4 and N2O emissions and removals-the magnitude
of which is highly uncertain-would have to be considered explicitly. There
are currently no reliable global estimates of these emissions and removals
for LULUCF activities. [1.2.2, 1.2.3,
3.3.2]